This invention relates to Direct Fired Absorption (DFA) Chillers and provides Turbo-Charging for reconcentrating an absorbent solution of environmentally acceptable water refrigerant in an aqueous salt solution with Lithium-Bromide or the like, it being a general object of this invention to increase the efficiency of the absorption process, particularly in two stage apparatus having primary and secondary generators for concentrating the absorbent refrigerant-salt solution.
The thermal reaction of water refrigerant is to absorb a large amount of heat when changing from a liquid to a vapor, and conversely to release the same amount of heat when the thermal reaction is reversed. The significant difference between mechanical refrigeration and absorption refrigeration is in the refrigerant used. That is, the costly typical fluorinated hydrocarbon compounds required by vapor compression refrigeration are replaced by a far less costly water/absorbent salt solution. The water refrigerant (distilled) with a latent heat of vaporization of approximately 1,000 Btu/lb is stable, non-toxic and readily absorbed, and is easily separated in large volume during the absorption process.
Significantly, absorption refrigeration requires a second fluid, an absorbent such as and preferably lithium-bromide, which is a non-toxic salt that has high affinity for water . This high affinity of lithium-bromide for the water refrigerant minimizes the volume of water solution that is pumped per pound of refrigerant per se transferred between the absorber and the generator of the apparatus, and its boiling point is also substantially higher than that of the water refrigerant, thereby facilitating separation of the refrigerant from the absorbent salt in solution.
The absorbent's affinity for water refrigerant changes with the amount of refrigerant present in the solution. A "dilute" absorbent is one with a high concentration of water refrigerant, and therefore has little affinity for it, and termed a "weak" absorbent. Whereas a "dense" absorbent solution is one with a low concentration of water refrigerant, and therefore has a great affinity for it, and termed a "strong" solution. An "intermediate" absorbent solution is a partially re-concentrated solution with moderate affinity. And a "mixed" absorbent solution is a mixture of intermediate and strong solution.
Direct Fired Absorption (DFA) Chillers are well known but have not been widely used in the United States. The DFA concept is based upon traditional absorption principles, similar to the principles of conventional vapor compression refrigeration and very similar to the concept principles of absorption chillers heated with steam and hot water. The direct fired heat absorption concept uses the environmentally compatible combination of water and an absorbent, and substitutes a burner for the conventional steam/hot water generator and auxiliary boiler. Direct fired absorption enjoys high efficiency COP's of 0.9 to 1.1, compared to 0.68 to 1.1 for said conventional two stage steam and/or hot water absorption. Furthermore, the boiler is eliminated and a burner only is required, or as compared to vapor compression refrigeration the burner replaces a mechanical compressor.
Direct fired absorption refrigeration is initiated when higher pressure liquid refrigerant solution from a condenser is discharged through an expansion device and into a lower pressure evaporator and collects therein. Expansion and "flashing" of the refrigerant solution occurs and said liquid refrigerant is collected in the sump of the evaporator. There is also the transfer of heat into the collected and cooled refrigerant solution, from comparatively warm system water that is pumped through a cooling tower causing the collected refrigerant solution to evaporate and resulting in vapor that is transferred to an absorber where it is absorbed into the lithium bromide refrigerant solution. This process not only creates a low pressure in the absorber that draws a continuous flow of refrigerant from the evaporator to the absorber, but also causes the vapor to condense as it releases the heat of vaporization in the evaporator. This heat together with the heat of dilution produced as the refrigerant condensate mixes with the absorbent is transferred into the cooling tower water and is carried out of the absorber. The assimilation of water refrigerant dilutes the lithium-bromide refrigerant solution and reduces its affinity for refrigerant vapor. In order to sustain the refrigeration cycle the solution is then reconcentrated by continuously pumping the dilute refrigerant solution from the absorber to a generator where heat is applied to distill the water refrigerant from the absorbent. As the water refrigerant is removed by distillation, the re-concentrated lithium-bromide refrigerant solution is returned through the absorber and to the generator to resume the absorption process, while the liberated refrigerant vapor from the generator is transferred to the cooler condenser and returned to its liquid state as the cooling tower absorbs the heat of condensation carried by the vapor. The liquid's return from the generator to the expansion device completes the cycle.
It is an object of this invention to increase the volume of water refrigerant applied to the system water evaporator, and to simultaneously strengthen the absorbent salt solution applied to the absorber that shares enclosure space with said water system evaporator. In practice, the system water evaporator and said absorber are combined and referred to herein as an evaporator-absorber means that collects low temperature weak absorbent. Accordingly, I provide a unique enhancer-concentrator unit that characterizes this invention and functions to increase the volume of water refrigerant and to simultaneously concentrate the weak absorbent subject to heat absorption into cooling tower water. It is weak absorbent salt solution that is collected in the sump of the evaporator-absorber means, and which is recirculated for evaporation therein and which is then re-processed by the enhancer-concentrator unit to increase the volume of the water refrigerant and to also strengthen the condition of the weak absorbent solution by concentrating it.
There are three basic types of double effect direct fired absorption systems, identified by their respective cooling solution flow cycles; namely, Series Flow, Parallel Flow and Reverse Flow. In each system the operating fluids are dilute (weak), intermediate, and concentrated (strong) lithium-bromide salt solutions, plus a refrigerant (water) vapor and liquid. The enhancer-concentrator unit of this invention is compatible with any one of the three basic types of system as it is employed in a closed weak absorbent circuit associated with the system water evaporator-absorber means, independent from either the first or second stage generator. However, heat energy is applied to said closed circuit from a strong absorbent generator. Accordingly, it is an object of this invention to provide an enhancer-concentrator unit for use with a system water evaporator-absorber means circuit of any one of the said three basic double effect direct fired absorption systems. The three basic systems are shown herein to provide chilled system water by operating in a refrigeration mode, however it is to be understood that this does not preclude providing hot water by operating in a heating mode when so required.
Direct fired absorption chillers are characterized by flow control means that reduce the pressure of strong absorbent solution produced by the regeneration that concentrates the lithium-bromide absorbent and vaporizes the water refrigerant. Reduction in pressure of the absorbent fluid is required for heat absorption and is usually accomplished by a flow control means in the form of an orifice, an expansion valve or pressure regulation device. Additionally however, it is an object of this invention to reduce and/or regulate the expansion of strong absorbent through the advantageous employment of work producing prime mover means for driving pump means used in the recirculation of absorbent solution to be separated into enhanced system water and concentrated absorbent solution in the enhancer-concentrator unit herein disclosed. It is another object of this invention to provide an energy recovery turbine as the prime mover means, operating on the strong absorbent pressure and simultaneously providing the pressure drop and expansion into weak absorbent for effecting the refrigeration function of the system.